1. Field of the Invention
The present invention is concerned with a method for immunizing animals of various types against Toxoplasma gondii. More particularly, it is concerned with such a method which makes use of a specific mutant of T. gondii which has surprisingly been found to give immunity without concomitant chronic infection of the animal.
2. Description of the Prior Art
Toxoplasmosis is an animal disease that can be transmitted to man, typically through contact with cat feces or raw meat. The disease is common to all domestic animals, including barnyard species. Ultimately, however, the possibility of human infection presents the most serious problem.
The spectrum of human disease due to Toxoplasma was characterized by a combination of serologic, immunologic and epidemological studies, and by isolation of Toxoplasma. In the acute infection where cells are destroyed by rapidly proliferating organisms, there may occur fever, pneumonia, an inflammation of the heart muscle, liver and skin (rash). Toward the end of this period or following a subclinical acute infection, localized or generalized swelling of lymph nodes is observed, especially in women. In newborn infants infected in utero, a subacute disease picture is typical. This results from the attenuation of the disease process by the passive immunization conferred by the mother, and the slow acquisition of active immunity due to immaturiy of the fetus and the newborn. In addition to the symptoms of acute Toxoplasmosis mentioned above, meningoencephalitis ("brain fever"), often with hydrocephalus ("water on the brain"), and retinochoroiditis (intraocular inflammation) are important. Most of the mothers who have given birth to infected babies had asymptomatic infections.
Thus, Toxoplasmosis deserves special attention because of the serious danger it raises for the unborn human baby. A pregnant woman may have the infection and unknowingly infect the fetus. Even if diagnosed and treated, the child may be born with permanent brain and eye damage. Diagnosis during pregnancy is uncertain at best and treatment uncertain and risky. For this reason, efforts to prevent infection during pregnancy are important.
It has been known in the past that mammals can be immunized against Toxoplasmosis. The procedure is very typical, and involves administration of a strain of the organism so that the mammal can build up immunity accompanied by specific antibodies against subsequent T. gondii challenge. This straightforward procedure presents a number of problems. For example, primary infection of cats (an important carrier of Toxoplasma) is usually followed by oocyst shedding before buildup of immunity. This phenomenon largely defeats the purpose of immunization, in that infective oocysts in the cat feces are a prime carrier of the disease. Furthermore, all known strains of the organism used for primary infections of mammals, while effective for purposes of building up immunity, tend to persist in the mammal for a long period of time, and possibly for a lifetime, with the result that the mammal is chronically infected. This in turn raises the possibility that if such a mammal becomes immunosuppressed later in life, the infection may reactivate with debilitating or even fatal results.
There is therefore a decided and heretofore unsatisfied need for a method of immunizing animals against T. gondii which on the one hand gives an adequate level of immunity, but on the other does not persist and cause chronic infection in the animal.